More than two gigawatts of wind power capacity has been installed offshore Europe in the past 20 years and the application of that experience is rapidly spreading to other parts of the globe. Differing environmental (loading) and geological (foundation resistance) conditions require that this experience related to foundation design and construction be extrapolated within a rational framework based on engineering mechanics. The vast global experience from the offshore oil and gas (O&G) industry can help guide the extrapolation of this experience.

The offshore O&G industry started in the Gulf of Mexico (GoM) off the coast of Louisiana in the late 1940s. More than 4,000 platforms have been installed in the GoM to date, with an additional 2,500 platforms installed worldwide. A wide range of fixed seabed substructure concepts have been constructed in water depths up to 600 meters for the offshore O&G industry, including steel jacket structures, concrete gravity base structures, tapered monopiles, guyed towers and compliant towers. Floating platforms have dominated in deeper waters up to 2,500 meters and include tension-leg platforms, semisubmersibles, spars and floating production, storage and offloading vessels.

Many of these fixed and floating O&G substructure concepts have been adopted by the offshore wind industry. Though shallow-water monopiles have thus far dominated construction practice for offshore wind turbines, as more offshore wind and marine energy projects come online, adoption of O&G geotechnical and geophysical expertise and experience will help the renewable energy sector develop both state-of the art techniques and a well-trained workforce.

Site Investigation
The first step for utilizing soil mechanics in the design and construction of foundations for offshore structures involves characterizing the magnitude and spatial variability of mechanical (strength, stiffness, compressibility, cyclic degradation) and hydrologic parameters, a process known as site investigation (SI).

A high-quality SI is an integral part of protecting and insuring investment in offshore wind energy, regardless of the type, scale and scope of the development. An SI should include assessments of geology, geohazards (i.e., landslides, natural gas) and measurements of local sediment engineering characteristics and their variability across the proposed site.

Mobilization and day-rate costs for onshore geotechnical SI equipment typically range from $1,000 to $5,000. In contrast, offshore SI costs may include a mobilization fee of $2 million and daily operational costs of $100,000 to $200,000. This two orders of magnitude shift in SI costs has made transition to offshore difficult for companies having experience in onshore wind turbine design, construction and financing.

These offshore exploration costs are relatively low when considering the $100 million to multibillion-dollar cost of a multiwell O&G project. However, they represent a significant fraction of total construction costs for an offshore wind farm when considering the typical price of an offshore wind turbine can be about $5 million for a monopile or $35 million for a jacket-supported turbine.

The high cost of seabed exploration relative to the total investment for an offshore wind development has the potential to lead to less thorough subsurface SIs and seabed models. This issue has directly resulted in delays and cost overruns for a number of wind projects constructed in European waters, where subsequent seabed investigations were required to augment previous studies.

The necessary number and depth of investigations will depend on the spatial extent of the farm, seabed variability (geological complexity) and subsurface layering of soft and stiff materials. In turn, this will control the cost of the SI program. Random, uniformly spaced investigations are likely not an optimum layout for an SI, and variability of the subsurface needs to be initially assessed through a desktop study followed by a geophysical investigation.

Marine geophysical tools are first used to image the seabed and identify geology, geohazards, obstructions and layering of various sediment types (i.e., soft versus stiff) both at and below the seabed.

The geophysical program will then be complemented by location-specific vertical-invasive investigations for assessment of soil mechanical properties. These investigations may consist of boreholes with soil sampling and subsequent laboratory testing, in-situ testing or a combination of the two methods.

In-situ test devices are electronic devices instrumented to measure penetration resistance, rotation resistance and often pore-water pressure. They significantly benefit SIs conducted in sediment, as they can provide a near-continuous (vertical) record over the depth of investigation. This record provides a large amount of high-quality information that can be used to assess changing sediment characteristics (strong versus weak, free-draining versus slow-draining), the extent of sediment variability (both vertically and laterally across a site), soil engineering parameters and foundation response.

In-situ testing is typically much quicker than drilling and sampling, which may lead to a more cost-effective investigation. However, in-situ testing cannot entirely replace the need for samples to confirm soil types and complement interpretations of engineering parameters. Many in-situ tests are not applicable for all sediment types, and gravelly soils, rocky or very hard materials that require special coring methods to overcome high penetration resistance require alternative testing methods. For these sediments and rocks, conventional offshore drilling and sampling techniques and geophysics remain central to SI practices.

Offshore Foundation Experience
The O&G industry has developed and used a wide range of foundation concepts. These can generally be separated into fixed or floating structures. Evolution of different foundation concepts and their use is dependent on seabed sediment conditions. This is best illustrated by comparing O&G developments in the GoM and the North Sea, which have drastically different geologic settings and development water depths.

The glacially formed sediments of the North Sea are highly variable and have led to the development and use of a wide range of foundation options. O&G structures in the North Sea were the first to use gravity base structures for both stiff and soft sediment conditions and the first to use suction caissons for both moored and fixed structures.

'The GoM's recently deposited and relatively uniform soft sediments have enabled advances in construction procedures for deepwater development and the utilization of advanced substructure concepts such as tension-leg platforms and spars. To date, all tension-leg platforms and about 55 percent of spars in the GoM are anchored with driven piles, while the remaining 45 percent of spars are anchored with suction caissons.

Offshore wind has generally followed the lead of offshore O&G, and experience from the O&G industry has been incorporated into offshore wind energy design standards (e.g., Høvik, Norway-based Det Norske Veritas' standard DNV-OS-J101). A similar range of foundation concepts has been used to support offshore wind turbines in water depths of less than 30 meters. Deepwater wind is currently in early development, and innovative foundation concepts are yet to be implemented into construction.

Offshore oil and gas pile design standards are geared toward foundation elements with a geometry typical of piles for jacket structures of the GoM and North Sea. Typical piles for offshore jackets are 20 to 135 meters long with a diameter of 0.6 to three meters. Foundation response is more strongly controlled by the ratio of length to diameter, or the slenderness ratio, which tends to range from a value of 20 to 130.

On the contrary, monopiles for offshore wind turbines typically have diameters of three to six meters and a slenderness ratio ranging from a value of five to 15.

Design standards have been calibrated predominantly for axial (vertical) capacity of driven piles in insensitive, highly plastic soft clays; very stiff to hard clays; and uniform dense sands'they have also been calibrated for lateral capacity of slender piles affixed to the substructure.

These American Petroleum Institute (API)—based design methods have been shown to be nonconservative for driven piles in soft clays of low plasticity or high sensitivity (for capacity), calcareous sands (for capacity), loose to medium-density sands (for capacity) and very dense sands (for installation). More recent analyses of the lateral capacity of the large-diameter slender piles typically used for fixed offshore wind turbines indicate that API-based methods conservatively neglect friction on the base of large-diameter monopiles and that methods to assess cyclic loading are inaccurate.

Offshore wind SIs and foundation design must go beyond conventional O&G practice, as differences in geometry and loading dynamics are significantly more important to design and power production efficiency. It should be noted that assessment of the natural frequency of a structure using conventional (Rayleigh) energy methods, which may have previously been applicable for flexible piles for offshore O&G structures, may overpredict the natural frequency of stout monopiles supporting offshore wind turbines.

Currently, offshore O&G foundation design is typically governed by ULS calculations, which might involve one-in-10,000-year or one-in-33,000-year cyclonic storm conditions. Failure mechanisms for different foundation types involve plastic yielding of the soil, which results from significant movements (sometimes meters) of the foundation element. When considering installation resistance, failure may occur under a differing mechanism than that which controls resistance to ULS conditions.

The performance of offshore wind turbine foundations during design loading is typically governed by SLS requirements. The tolerance of the nacelle movements might be so small that displacements of the foundation have to be within millimeters during normal loading conditions. Soil strength will still control installation resistance.

This difference in design criteria might lead to misunderstandings between the two disciplines and different soil parameters might be necessary for a high standard of design. The earlier in a project it can be established which criterion is governing, the better it will be for the project.

Offshore Wind Moving Forward
As offshore wind and other renewable ocean-energy technologies play a more prominent role in energy portfolios worldwide, projects will increase in both scale and complexity, requiring some of the expertise and experience the O&G industry has developed over the past several decades.

Offshore wind requires qualified specialists in geotechnical engineering and marine geology and geophysics; ports and facilities for vessels, offshore infrastructure and foundation manufacturing and deployment; vessels seaworthy for expected metocean conditions that can accommodate varying SI and foundation installation activities; local or regional companies with experience and equipment to conduct integrated geophysics, sampling and in-situ testing portions of offshore SIs; and companies with experience and equipment to perform high-quality laboratory testing of soil parameters and model-scale studies of sediment-foundation interaction.

This increase in demand also provides an opportunity for partnerships between industry, educational institutions and manufacturers to provide state-of-the-art facilities to develop a well-trained workforce. Economic benefits associated with new developments in offshore SI methods and interpretation as well as foundation concepts and design procedures will follow.

Acknowledgments
The authors would like to thank Tom McNeilan and Kevin Smith of Fugro Atlantic (Norfolk, Virginia) for support during the research conducted in relation to this article.

James Schneider, an assistant professor at University of Wisconsin, Madison, has worked with geotechnics and surveying company Fugro West on site investigation and foundation design for coastal and offshore structures. He is also an external consultant on proposed offshore wind farms along the U.S. East Coast and Great Lakes regions for Fugro Atlantic.

Melissa Landon Maynard is an assistant professor of civil engineering with an expertise in geotechnical engineering at the University of Maine. She is currently a principal investigator for the Department of Energy's DeepCwind Consortium's validation of coupled models and optimization of materials for offshore wind structures.

Marc Senders is currently a principal geotechnical engineer who has been involved in numerous site investigations and foundation designs for offshore structures during his 17-year-career. His Ph.D. thesis at The University of Western Australia covered suction caissons in sand as tripod foundations for offshore wind turbines.

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